Devices for compensating magnetic fields are known. Such devices generally use a feedback control loop, wherein an interference field amplitude is measured by one or more sensors. Having been processed by means of a controlling device, the measured signal is passed as a control signal to Helmholtz coils, the locations of which minimize the interference field amplitude at the spot of the sensor by emitting a magnetic compensation field.
The magnetic field to be compensated may be the terrestrial magnetic field, or may be generated by other devices in the surroundings.
Feedback systems working only digitally may also be used besides feedback systems working only analogically. In order to avoid the disadvantages of such systems, hybrid systems were also developed.
Thus, the applicant's patent EP 1 873 543 A1 describes such a hybrid system for compensating magnetic fields. With this system for measuring the magnetic field, a combination sensor is used, which has coil sensing elements as well as flux gate sensors. The signals of the coil sensing elements are used in this connection for the analogue part of the controlling, the signals of the flux gate sensor for the digital part.
With coil sensing elements, the voltage induced by an external magnetic field in a coil is measured.
There are biaxial and triaxial flux gate sensors. Such sensors may measure magnetic fields perpendicular to a plane, or in all three directions in space. In doing so, three coils each are on two ferromagnetic cores. A triangle generator generates a current flowing through the excitation coils. A magnetic field is created in the ferromagnetic cores by this current. The change of the magnetic flux induces voltages in the so-called pick-up coils. If there is no external field, the difference of the voltages of the pick-up coils is zero. If an external field is created, a difference voltage is generated.
Such hybrid systems with an analogue and a digital control loop have the disadvantage that coil sensing elements as well as flux gate sensors have to be placed in one housing. In doing so, the physical proximity of the flux gate sensors to the coil sensing elements causes crosstalk of the chopper frequency to the coils, and therewith to undesired frequency components in the coil signal. In this context, the chopper frequency can be conceived of as the frequency with which the excitation coils of the flux gate sensor are operated. Therefore, the individual sensors have to be accurately arranged inside the housing, in order to keep this crosstalk small. Nevertheless, a structural shape of approximately 2 cm×2 cm×2 cm and a weight of 300 g can hardly be matched.